According to known methods, alkali metal hypohalites may be produced by electrolysis of an alkali metal brine (e.g. sodium chloride) in diaphragmless electrolysis cells in which the electrolyte is flowed one or more times through a series of cells having anodes and cathodes between which the alkali metal brine is electrolyzed. The halogen (e.g. chlorine) is discharged at the anode according to the reaction: EQU 2Cl.sup.- .fwdarw.Cl.sub.2 +2e.sup.-
while water is reduced at the cathode with evolution of hydrogen and formation of sodium hydroxide according to the reaction: EQU 2Na.sup.+ +2H.sub.2 O+2e.sup.- .fwdarw.2NaOH+(H.sub.2).uparw.
The halogen (e.g. chlorine) reacts with the alkali metal hydroxide to form hypochlorite according to the reaction: EQU Cl.sub.2 +NaOH.fwdarw.NaClO+NaCl+H.sub.2 O
The sodium hypochlorite dissolved in the solution may react to form hypochlorous acid, according to the equilibrium: EQU NaCl+H.sub.2 O.revreaction.HClO+Na.sup.+ +OH.sup.- ( 1)
The hypochlorous acid, in turn, partially dissociates into hydrogen ions and hypochlorite ions according to the equilibrium: EQU HClO.revreaction.H.sup.+ +ClO.sup.- ( 2)
The equilibrium constant of both reactions (1) and (2) depends upon the pH of the solution. For example, at pH values less than 5, all of the active chlorine is present as hypochlorous acid and hypochlorite ions whereas at high pH values, nearly all the active chlorine is present as hypochlorite ions. Therefore, active chlorine concentration is usually referred to, although it comprises molecular chlorine, hypochlorous acid and hypochlorite ions.
In the electrolysis cells used for generating hypochlorite solutions, the pH of the solution is usually kept above 7.5 so that nearly all the active chlorine is present as hypochlorite ions. Moreover, the temperature is kept low enough (generally lower than 35.degree. C.) to prevent dismutation of hypochlorite to chlorate and the brine is rather dilute and generally contains from 20 to 40 gpl of chloride ions with sea water often being used as the electrolyte. The concentration of active chlorine (that is hypochlorite ions) in the effluent is generally lower than 2-3 gpl.
Higher concentrations of hypochlorite are possible only at a cost of prohibitive current efficiency losses. In fact, the cathodic reduction of hypochlorite to chloride is favored over the reduction of water from a thermodynamical standpoint and therefore, it is highly competitive with respect to hydrogen evolution. With known cells, the practical maximum hypochlorite concentration cannot be higher than 8-10 gpl. Beyond these limits, the current efficiency comes to naught since the hypochlorite ions are reduced at the cathode as fast as they are formed.
The most serious problem in the known cells for direct sea water chlorination, or chlorination of brines prepared from raw salts and water stems from the fact that calcium and magnesium, and to a lesser degree other alkaline earth metal and alkali metals, which are always present in large amounts as impurities in raw salt or in sea water, precipitate as hydroxides on the cathodes generating scale thereon which before long fills the interelectrodic gap. Periodic washing of the cells with acidic solutions, such as hydrochloric acid solutions, is the only effective way of maintaining a continuous operation and such washings are carried out at regular intervals, varying from some days to one or more weeks depending on the quality of the salt used and/or the operating conditions of the plant.
In plants with a power production above a certain minimum, a fixed, integrated washing system is provided and fixed washing systems, besides obvious complications and additional expense costs for a chlorination plant, require the choice of suitable materials which are non-corrosive to the washing agents used. For example, the cathodes must be made of materials sufficiently resistant to hydrochloric acid to withstand frequent washings and the use of titanium or other valve metal cathodes is common practice which obviously entails higher costs and a higher hydrogen overvoltage. Moreover, repeated acid washings reduce the average operating life time of titanium anodes coated with a surface layer of electrocatalytic, non-passivatable materials. The titanium base, in fact, tends to lose its electrocatalytic coating as a result of the acid attacks which produces corrosion thereof.
In alkali metal chlorate production, electrolytic cells similar to those used in producing hypochlorite are utilized, but the working conditions are such that the dismutation of hypochlorite and/or hypochlorous acid to chlorate is favored whereby the current efficiency loss due to cathodic reduction of hypochlorite is reduced. Therefore, the temperature of of the electrolyte is kept around 60.degree.-90.degree. C. and the pH is kept below 3-4 by adding hydrochloric acid. The electrolyte flows in a circuit comprising the electrolysis cell and a holding tank to reduce the residence time within the cell and to allow hypochlorite dismutation to chlorate in the holding tank before feeding the electrolyte back into the cell.
In both instances, means are used to prevent the hypohalite generated within the solution from diffusing towards the cathode. For example, the solution is passed through the cell at a high speed with a short residence time therein while keeping the flow of electrolyte between the electrodes as laminar as possible and then into a holding tank. The hydrogen bubbles present in the electrolyte produce a certain turbolence, especially in proximity to the electrodes, which enhances the diffusion of the hypohalite ions towards the cathode by convective mass transfer.
Although brine electrolysis is a highly advanced technical field of great industrial importance and a constant research effect is exerted and wherein the importance of technical improvements is substantial, the process of the present invention has never been practiced nor have the advantages therefrom been secured.